ABSTRACT Complement is a group of 50-60 soluble and membrane-bound proteins that constitutes the first line of defense of the intravascular space. Complement is also a principal cause of tissue damage, with the complement alternative pathway (AP) in particular being implicated in numerous disease and injury states. Complement activation is driven by the C3 convertases, serine proteases assembled at sites of C activity in a multi-step process. Assembly of the AP C3 convertase requires several components. One of them, properdin, is a particularly attractive therapeutic target: Properdin is AP-specific and its inhibition ameliorates disease in several mouse models. Moreover, properdin-deficient individuals are well except for an increased susceptibility to meningococcal disease, a condition that can be addressed by vaccination. The Ixodes ticks are blood-feeding arachnids. During their blood meals they are exposed to the immune defenses of their hosts. Ixodes ticks produce salivary proteins that inhibit the AP pathway by blocking properdin activity. Ixodes scapularis salivary protein 20 (Salp20) is a well-characterized representative of this family. Salp20 binds properdin, disrupting convertase formation, and dissociates AP convertase by releasing properdin from the complex. Salp20 family proteins can inhibit AP activity in multiple mammalian systems suggesting that the Salp20 family targets a highly conserved and vulnerable properdin site. Our long-term goal is to promote complement-based therapy, in this case facilitate the development of a small molecule inhibitor of properdin. Salp20 itself would be expected to be highly immunogenic and therefore inappropriate as a therapeutic. A small molecule that recapitulates Salp20 anti-properdin activity would be an attractive alternative. In order to develop a Salp20-based strategy for the therapeutic inhibition of AP-dependent complement activity, we propose the following SPECIFIC AIMS: 1. Elucidate the Salp20/properdin site of interaction. We will use employ ?protein painting?, structural models and site-directed mutagenesis to delineate the Salp20/properdin contact regions. 2. Develop candidate small molecules that mimic Salp20 activity by targeting its properdin attachment site. We will employ next-generation and standard phage display methodologies to identify candidate peptides that bind properdin at the Salp20 attachment site and in vitro AP inhibition assays to assess their biochemical activities. 3. Use ex vivo models to elucidate the consequences and mechanism(s) of Salp20 activity and evaluate the functional activity of candidate peptides developed in SA#2. These aims will provide key information and materials for the development and usage of a novel Salp20-like anti-properdin therapeutic and clarify the role of properdin and the impact of Salp20 on activated platelets, neutrophils, and the interactions between these cell types.